Precipitated barium sulfate is a known pigment, which can be used as a filler in the rubber industry, as an opacifier in art printing and photographic papers and in the plastic industry, and as a brightening agent and a fiber-dulling agent in the textile industry.
Barium sulfate is produced in known manner from water-soluble barium salts and water-soluble sulfates by a wet chemical precipitating process. These conventional precipitating processes involve a more or less time-consuming crystallization from isothermally supersaturated solutions.
Owing to the very low solubility of the precipitates the crystals are very small and the conversion almost always amounts to about 100%. The optimum properties of pigments and extenders from the aspect of application technology will be achieved with certain particle size distributions. For this reason it is by no means sufficient to form the desired barium sulfate precipitate in a prescribed chemical composition but the wet chemical process must be controlled to result in the desired primary particle size.
Precipitated barium sulfate is used as an inert pigment in numerous fields. Known fields of application include the use of the precipitated barium sulfate as a filler in the dyestuff and paint industry, in the plastic and rubber industries, or the use as a dulling agent in the production of fibers.
In these fields the barium sulfate is substantially used as an inert white filler based upon the fact that it is substantially inert chemically and physically.
In other applications, the "filler" is required to have functional properties in order to impart certain property profile to the system in which the filler is used. This is generally accomplished with so-called active fillers, such as silicates, oxides, carbon black, sulfur, which may be of natural, mineral origin or may be synthetically produced.
All so-called active fillers are characterized by having a surface reactivity owing to the presence of esterifiable hydroxyl groups, reducible carbonyl or carboxyl groups or radical-splittable molecules (e.g. polysulfides).
Barium sulfate lacks such a surface activity. In the (SO.sub.4).sup.2- tetrahedron, the central atom has its highest oxidation number. The high charge density of the Ba.sup.2+ ions ensures a saturation of the bond sites on the crystal surface by the Ba.sup.2+ ions.
A known measure of providing surface reactivity consisting of an aftertreatment with inorganic and/or organic adjuvants, such as a silanization, will not produce the desired results with barium sulfates because, by contrast with other oxide pigments, such as TiO.sub.2, ZnO and SiO.sub.2, and by contrast also with carbon a particle of BaSO.sub.4 has no surface centers which can be esterified and/or complexed and the aftertreating agents cannot be fixed with formation of a covalent bond.
A strictly physical-mechanical application, such as the application of additional components by a simple admixing, will not produce the desired result. When coated barium sulfate is incorporated in a polymer system, the aftertreating agent bonded by physical sorption will be detached from the surface. In order to prevent a subsequent detaching, the aftertreating agent must be chemically fixed to the BaSO.sub.4 surface by a covalent bond or must be electrostatically fixed to said surface.
Published German Application No. 28 50 609 discloses stable filler-polyol dispersions in which the filler consists of a coprecipitate of barium sulfate and silica. The coprecipitate has an average particle size of 70 .mu.m.
The phosphate-containing pigments known from Published German Application No. 17 17 096 are produced by coprecipitation in that sulfate ions and hydrogen phosphate ions are added to a solution which contains barium ions. The precipitation is effected at temperatures from 0.degree. to 100.degree. C.
Published German Application No. 33 47 191 discloses a process of producing a surface-treated barium sulfate which has an improved dispersibility in resin compositions. In that known process an aqueous solution of an alkali silicate is added to an aqueous suspension of barium sulfate, which suspension contains a surplus of barium ions. As a result, barium silicate is precipitated on the surface of the barium sulfate. This is succeeded by a treatment with mineral acid for decomposing the barium silicate to form a water-containing or hydrated silica. The product may subsequently be treated with a silane coupling agent.